Diffusion transfer photographic process utilizing development restrainers



United States Patent 3,265,498 DEFFUSION TRANSFER PHQTOGRAPHIC PROC- ESS UTILIZING DEVELOPMENT RESTRAINERS Howard G. Rogers, Weston, Mass, and Harriet W. Lutes,

Cape Elizabeth, Maine, assignors to Polaroid Corporation, Cambridge, Mass, a corporation of Delaware No Drawing. Filed Aug. 22, 1960, Ser. No. 50,849 18 Claims. (Cl. 96-3) The present invention is concerned with photography and, more particularly, with color diffusion transfer processes.

One object of the present invention is to provide processes and products for improving the color quality and the density of diffusion transfer images produced by diffu sion transfer processes employing dye developers.

Another object of this invention is to provide processes and products useful in diffusion transfer processes, whereby further development of developable silver halide is restrained, and in a preferred embodiment, undeveloped developable silver halide is rendered substantially undevelopable, after a predetermined period, thereby providing diffusion transfer images of improved density and color quality.

The invention accordingly comprises the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and the products possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

US. Patent No. 2,983,606, which issued May 9, 1961, on the copending United States application of Howard G. Rogers, Serial No. 748,421, filed July 14, 1958, which isa continuation-in-part of United States application, Serial No. 415,073, filed March 9, 1954 (now abandoned), discloses and claims diffusion transfer processes wherein dye developers, -i.e., a compound which is both a silver halide developing agent and a dye, are used to form color images by diffusion transfer processes. As disclosed therein, an exposed photosensitive silver halide emulsion is developed in the presence of a dye developer and an imagewise distribution of unreacted, unoxidized dye developeris formed as a function of, and under the control of, the development. At least a portion of this imagewise distribution of mobile, unreacted dye developer is transferred, by imbibition to a superposed image-receiving layer to create thereon a positive dye image.

In a particularly useful embodiment of such diffusion transfer processes, a photosensitive element containing a silver halide emulsion and a dye developer is exposed and a liquid processing composition is applied thereto, for example, by immersing, coating, spraying, flowing, etc., in the dark. The exposed photosensitive element is superposed prior to, during or after the application of said liquid processing composition on a sheetlike support element which may be utilized as the image-receiving layer. In a preferred embodiment, the photosensitive element contains the dye developer in a stratum positioned behind the silver halide emulsion layer with respect to the incident actinic light, and the liquid processing composition is applied to the photosensitive element by being spread in a relatively thin layer as the photosensitive element is brought into superposed relationship with an image-receiving element. While the dye developer may be utilized in the processing composition where monochromatic images are desired, use of a negative containing a plurality of silver halide emulsions for forming multicolor images requires that at least the dye developers employed in the development of the innermost emulsion layer or layers be initially contained in the photosensitive element. Where the negative is .a multilayer negative, the dye developer employed with the outermost photosensitive emulsion layer may, if desired, be contained in the processing composition, although in a preferred embodiment, the dye developer associated with the outermost emulsion layer is also contained in the photosensitive integral multilayer element. Since the outermost emulsion layer is customarily a blue-sensitive emulsion, use of a yellow dye developer in a layer behind the blue-sensitive emulsion makes possible the omission of the conventional yellow filter layer required to facilitate the desired selective exposure of the underlying redand green-sensitive emulsion layers. Products and processes for effecting the formation of multicolor images by the use of integral multilayer negatives are disclosed and claimed in the copending United States application of Edwin H. Land and Howard G. Rogers, Serial No. 565,135, filed February 13, 1956. The previously mentioned copending United States application Serial No. 748,421 (now US. Patent No. 2,983,606, issued May 9, 1961) also contains a disclosure of the formation of multicolor images using dye developers contained in an integral multilayer negative.

Reverting once again to a description of the process, the liquid processing composition permeates the emulsion to initiate development of the developable silver halide. The dye developer associated with each of the silver halide emulsion layers is immobilized or precipitated in the developed areas as a consequence of this development. This immobilization is apparently, at least in part, due to a decrease in the mobility or solubility of the oxidation product of the dye developer as compared with the unoxidized dye developer. This immobilization may also be due in part to a localized reduction in the alkali concentration as a function of the development. The unreacted dye developer associated with undeveloped and partly developed areas of the emulsion is diffusible, and thus an imagewise distribution of mobile unoxidized dye developer is formed as a function of the point-to-point degree of exposure and development of the silver halide emulsion. At least part of this imagewise distribution of diffusible, unoxidized dye developer is transferred by imbibition to the superposed image-receiving layer or element, said transfer being effected sufficiently to the exclusion of the less mobile oxidized dye developer to create a visible transfer image on the image-receiving layer.

It will be appreciated that processes of this type employing an integral multilayer negative processed with a common processing composition necessarily require that unoxidized dye developer diffusing from an underlying emulsion to the superposed image-receiving layer must pass through at least one other overlying photosensitive silver halide emulsion. If the unoxidized dye developer diffusing from the underlying layer enters an area of the overlying emulsion containing developable silver halide, there is as much likelihood that the diffusing dye developer from the underlying emulsion will react as that the dye developer associated with said overlying emulsion will react. As an illustration of the problem involved, reference may be made to a photosensitive integral multilayer element containing red-sensitive, greensensitive and blue-sensitive silver halide emulsions coated in that order on a common support. These silver halide emulsions have associated therewith, respectively, a cyan dye developer, a magenta dye developer, and a yellow dye developer either in a layer behind the emulsion or in the emulsion layer. Assuming that, in a given unit area of said negative, exposure is effected only of the green-sensitive emulsion, unoxidized cyan dye developer will eventually diffuse into and through such exposed areas of the green-sensitive emulsion en route to the imagereceiving layer superposed over the outermost or bluesensitive emulsion layer. If the magenta dye developer has substantially completed the development of the developable green-sensitive silver-halide prior to the arrival of the unoxidized cyan dye developer, no harm will be done. On the other hand, if the migrating cyan dye developer diffuses into the green-sensitive emulsion layer while appreciable undeveloped developable greensensitive silver halide is still present, said cyan dye developer may react since it will not distinguish between developable silver halides of different color sensitivity. This reaction in the wrong silver halide emulsion may be referred to as cross-talk, and manifests itself in producing transfer images having reduced color separation and in the instance given, less cyan density and probably more magenta density than the actual exposure would indicate should be present in the transfer image.

As previously noted, it is a principal object of this invention to provide improved color separation and increased density in color transfer images obtained by diffusion transfer processes, particularly those employing dye developers. This object is accomplished in accordance with this invention by causing the developable silver halide remaining undeveloped after a predetermined period to be rendered undevelopable, so that unoxidized dye developer diffusing through said emulsion layer will not be immobilized by development of developable silver halide contained therein.

It will be recognized that it is necessary that development of the developable silver halide be allowed to continue in the normal manner at least for a length of time sufficient to modulate the associated dye developer, i.e., to give the requisite imagewise distribution of the appropriate dye developer. It has been found in practice that suflicient development to modulate the dye developer may be effected in time periods as short as five to twenty seconds, even though development, if unimpeded, would continue for a substantially longer time. We have found that this control may be accomplished by incorporating in a layer of either the photosensitive element or the image-receiving element, or in some instances in the processing composition, a reagent which is made available to the developable silver halide of a given emulsion layer only after a predetermined period during which development is effected without interference by said reagent. Since this added reagent effectively restrains, i.e., minimizes, further development of developable silver halide after this predetermined period, suitable reagents employed [for this purpose may be referred to as development restrainers. While such reagents frequently will have characteristics similar to reagents commonly referred to as antifoggants, they perform a function herein different from what is normally contemplated as the function of an antifoggant, i.e., their function is not to reduce the fog density in unexposed areas, although under some circumstances, they may also perform this function to a small degree. In some in stances, a given reagent may be used in a small concentration in the processing composition wherein it functions as an .a-ntifoggant, and an additional quantity is contained in the photo-sensitive element or in the image-receiving element in a form whereby it is released only after a predetermined period whereupon it acts as a development restrainer.

Rea-gents which are particularly suitable for use as development restrainers are those which will form products or complexes with undeveloped silver halide, whether exposed or unexposed, but at least with exposed silver halide, which products or complexes are substantially less developable by a silver halide developing agent, e.g., by the dye developer, and which preferably are substantially insoluble, and hence essentially undevelopable, i.e., developable only with difficulty. In a preferred embodiment, the desired predetermined period during which development is effected without interference is accomplished by incorporating the development restrainer in a chemical form or in a physical location such that its availability to the developable silver halide is limited or restricted, e.g., as a result of the distance through which it must diffuse to reach the developing silver halide, or as a consequence of a significantly lower diffusion rate than said developing agents. Where the development restrainer is at least initially substantially slower in difliusion than the dye developer, as for example, as a result of the inclusion of a relatively long chain alkyl group, the development restrainer may be positioned in a layer of the multilayer negative, e.g., in the dye layer containing the dye developer or in an interlayer positioned on the side of the dye layer remote from the silver halide emulsion layer. A relatively more diffusable development restrainer may be used provided that it is located sufficiently remote physically from the silver halide emulsion layer upon which it is to act, as by incorporation in the image-receiving layer, i.e., in the image-receiving layer itself or in a layer over the image-receiving layer or in a subcoat beneath the image-receiving layer. Such development restrainers may also be incorporated in a layer in the photosensitive element provided said restrainer is present in such a form as to be only slowly dissolved by the processing composition. Under some circumstances, the development restrainer may be similarly contained in a coating over the outermost or blue-sensitive silver halide emulsion layer.

Development restrainers suitable for use in the processes of this invention will readily suggest themselves to one skilled in the art. By way of illustration, mention may he made of long chain :thiobarbituric acids, e.g., dodecyl thiobarbituric acid, and phenyl-mercaptotetrazoles, e.g., l-phenyl-5-rnercaptotetrazole.

In a preferred embodiment of the invention, the development restrainers .are employed, in combination, with an ant-ifoggant initially contained in the processing composition and/ or in a layer of the photosensitive element so as to be available for action relatively promptly after the application of the processing compositon. Such antifoggants perform the usual function of an antifoggant, i.e., they minimize the development of silver fog in unexposed and partially exposed areas of the several photosensitive silver halide emulsions and increase the differentiation between exposed and unexposed silver halide by the developing agent. As previously noted, the development restrainer may and frequently is a compound of the broad class of reagents frequently referred to as antifoggants. An essential distinction between the use of a compound as an antifoggant and as a development restrainer as contemplated by this invention is in the time sequence in which it is available to act upon the silver halide emulsion layer. Furthermore, many useful development restrainers may be found among compounds whose action upon silver halide is too vigorous to be effectively employed as anti-foggants, since they would tend to reduce density in the areas intended to be developed. Since it is the purpose of the development restrainers in the instant invention to render the residual developable silver halide substantially undevelopable after a predetermined period of development, we may successfully use a reagent which acts nonimagewise in rendering the remaining undeveloped silver halide, exposed and unexposed, undevelopable by the silver halide developing agent or agents present.

In a preferred embodiment, development is effected in the presence of one or more auxiliary or accelerating silver halide developing agents as also disclosed and claimed in the previously mentioned copending UnitedStates application Serial No. 748,421. Such auxiliary silver halide developing agents may initiate development by virtue of their relatively greater diffusion rate than the larger dye developer molecule. In at least some instances, the auxiliary developer is believed to develop a portion of the developable silver hali e, the dye developer being oxidized by reaction with the oxidation product of the auxiliary developing agent. As examples of suitable auxiliary developing agents, mention may be made of benzenoid silver halide developing agents, i.e., silver halide developing agents containing a benzene or naphthalene nucleus substituted by at least two hydroxyl, amino, and/or alkylamino radicals so as to be capable of developing exposed silver halide and forming a quinonoid oxidation product, e.g., to'luhydroquinone, phenylhydroquinone, 4'-methylphenylhydroquinone, etc. Another particularly useful class of auxiliary developing agents is S-pyrazolidones, and preferably 1-phenyl-3-pyrazolidone, and 1-phenyl-4,4- dimethyl-3-pyrazolidone. It will be understood that reference to development of developable silver halide is in tended to include development by the auxiliary developing agent as well as by the dye developer itself. In a particularly useful embodiment, an auxiliary developing agent is contained in a layer over the outermost silver halide emulsion, in accordance with the disclosure of our copending United States application, Serial No. 50,850, tiled August 22, 1960.

The following example illustrates the use of a development restrainer incorporated in an underlying layer of the photosensitive element in a manner whereby its availability to the developing silver halide emulsion is delayed by .a predetermined period, e.g., by the time required to dissolve the development restrainer and, following dissolution, to diffuse to the silver halide. This example also illustrates that a reagent normally used as an antifoggant may also be useful as a development restrainer when used in appropriately larger concentrations.

Example 1 A photosensitive element was prepared by coating a gelatin-swbcoated cellulose acetate film support with the following coating solutions; proportions being by weight unless otherwise stated:

(1) A tetrahydrofuran-acetone solution (1 to 1 by volume) comprising 5.5% of 1,4-bis-[B-(2',5'-d-ihydroxyphenyl)isopropylamino]-anthraquinone (a cyan dye developer), about 0.8% of Resoflex 296 (trade name of Cambridge Industries Co., Inc., Cambridge, Mass, for an alkyd resinous plasticizer), 0.4% S-nitrobenzirnidazole, and 2% cellulose acetate hydrogen phthalate.

(2) A red-sensitive silver halide emulsion.

(3) An acetone solution comprising 2% cellulose acetate hydrogen phthalate, 0.75% cellulose acetate and 0.05% S-nitrobenzimidazole.

(4) A 2% aqueous polyvinyl alcohol solution.

(5) A tetrahydrofuran-acetone solution (1 to l by volume) comprising 3.5% of 2 [p (2,5 dihydroxyphenethyl) phenylazo] 4-np ropoxy-l-naphthol (a rn agneta dye developer), 2% of cellulose acetate hydrogen phthalate and 0.4% of Resoflex 296.

( 6) A greenasensitive silver halide emulsion.

(7) An acetone solution comprising 2% cellulose acetate hydrogen phthalate and 0.5% cellulose acetate.

(8) A 2% aqueous polyvinyl alcohol solution.

(9) A tetrahydrofuran solution comprising 3% of lphenyl 3 N n hexyl oarbamyl 4 [p (2',5' dihydroxyphenethyl)-phenylazo]-5-pyrazolone (a yellow dye developer), 2% of cellulose acetate hydrogen phthalate and 0.4% Resoflex 296.

(10) A blue-sensitive silver halide emulsion.

The above photosensitive element was exposed and then processed by spreading an aqueous processing solution comprising:

I Percent 1-phenyl-3-pyrazolidone 1.2 2,S-bis-ethyleniminohydroquinone 0.9

S-nitrobenzimidazole 0.125

between said photosensitive element and an image-receiving element as said elements were brought into superposed relationship. The image-receiving element comprised a cellulose acetate subcoated baryta paper which had been coated with an ethanol solution containing 4% of N-methoxymethyl polyhexamethylene adipamide. After an imbibition period of approximately three minutes the image receiving element was separated and contained a positive image of the photographed subject, having improved photographic quality as compared with the image obtained using a similar negative but omitting the S-nitrobenzimidazole from the cyan dye layer.

As previously noted, the development restrainer may be incorporated in the image-receiving element. The following examples illustrate this manner of practicing this invention:

Example 2 A sheet of subcoated baryta paper was coated with an aqueous solution containing a small amount of acetic acid and comprising:

Percent Elvanol 72-60 (trade name of E. I. du Pont de Nemours & Co., Wilmington, Delaware, for high viscosity, 100% hydrolyzed polyvinyl alcohol) 2.4 Poly-4-vinylpyridine 2.4 l-phenyl-S-mercaptotetrazole 0.08

Example 3 An image-receiving layer was prepared as described in Example 2, except that l-phenyl-S-mercaptotetrazole is omitted from the layer of polyvinyl alcohol and poly-4- vinyl pyridine and is contained in a subcoat between the support and said polyvinyl a-lcohol/poly-4-vinyl pyridine receiving layer. This subcoat may be prepared using a methanol solution containing 3.5% of l-phenyl-S-mercap- .totetrazole and 1.2% of partially hydrolyzed polyvinyl acetate (Vinylite MA-28l8). The transfer images obtained using the image-receiving elements of Examples 2 and 3 show increased maximum density .and improved color separation; in some instances an increase in film speed (exposure index) was also observed with the imagereceiving element of Example 3 as compared with that of Example 2.

Example 4 An image-receiving layer was prepared by coating a cellulose acetate subcoated baryta paper with a layer containing 4% N-methoxymethyl polyhexamethylene adiparnide in an ethanol solution. A 40% aqueous ethanol solution containing 2% thioacetanilide was then applied to the surface of this image-receiving layer. The image-receiving layer was then dried and employed in a diffusion transfer process using a photosensitive element such as that described in Example 1. The resulting transfer image exhibited increased density and better color separation than if the thioacetanilide were omitted from the image-receiving layer.

In each of the above examples, the resulting color transfer image exhibited increased maximum density and improved color separation.

Other materials useful as the image-receiving layer are disclosed in the oopending United States applications referred to herein, and include polyvinyl alcohol without a mordant gelatin, gelatin containing a mordant such as poly-4-vinyl pyridine, etc.

The multicolor negatives may employ the dye developer molecularly dispersed in a layer of alkali-permeable plastic, or the dye developer may be dissolved in a water-immiscible solvent and dispersed in gelatin to provide the desired dye layer. Similarly, suitable interlayers for the photosensitive element include polyvinyl alcohol or gelatin of appropriate thickness. The use of a .gel-atin inter-layer is particularly advantageous where the dye developer is dispersed in gelatin to provide the dye layer.

The processes of this invention are applicable to dye developer processes employing onium compounds, e.g.,

quaternary ammonium, quaternary phosphonium, and tertiary sulfonium compounds, as disclosed and claimed in the copending United States application of Howard G. Rogers and Milton Green, Serial No. 50,851, filed August 22, 1960, now US. Patent No. 3,173,786, issued March 16, 1965.

Use of the development restrainers contemplated by this invention is also particularly efficacious with photosensitive multilayer negative-s having an auxiliary developer incorporated in a gelatin layer over the outermost or blue-sensitive silver halide emulsion layer. Photosensitive elements of this type are disclosed and claimed in our copending United States application, Serial No. 50,850, filed August 22, 1960, now US. Patent No. 3,192,- 044, issued June 29, 1965.

In addition to the illustrated use of S-nitrobenzimidazole, mention may be made of S-methylbenzimidazole, Z-aminobenzimidazole, thioacetanil'ide, Z-mercapto-benzo- .thiazole and benzot-riazole as being particularly useful antifoggants which may be incorporated in the photosensitive element. Where the development restrainer is incorporated in the image-receiving element, l-phenyl-S- mercaptotetrazole has been found to be particularly useful.

It has also been found to be desirable in certain instances to include a silver halide solvent, e.g., sodium thiosulfate, in the processing composition; improved color separation has been observed where such a silver halide solvent has been used in the absence of a silver precipitating agent in ,the image-receiving layer.

The dye developers which are used in the processes of this invention as noted above are compounds which contain in the same molecule the chromophoric system of a dye and also a silver halide developing function. Preferred dye developers are those comprising hydroxy substituted benzenoid silver halide developing radicals and especially dyes containing hydroquinonyl and catechol silver halide developing radicals. Representative dye developers for use in the processes of this invention are disclosed in the previously mentioned copending United States applications, Serial Nos. 748,421 (now US. Patent No. 2,983,606, issued May 9, 1961), 565,135 and other copending applications referred to therein.

The aqueous processing compositions used in the processes of this invention are alkaline in nature and preferably should have a pH of at least 12. Particularly suitable materials for rendering the processing composition alkaline are sodium hydroxide, potassium hydroxide, diethylamine, etc. As previously noted, the processing composition preferably includes .an antifoggant which is available essentially immediately upon permeation of the photosensitive silver halide layer by the processing composition. The processing composition may also contain other reagents such as the auxiliary developing agent, stabilizers or preservatives, antioxidants, organic solvents which preferentially increase the solubility of the unox'idized dye developer in the processing composition, etc.

The process of this invention are especially useful in composite film units intended for use in a Polaroid Land Camera, sold by Polaroid Corporation, Cambridge 39, Massachusetts, or a similar camera structure such, for example, as the camera forming the subject matter of US. Patent No. 2,435,717, issued to Edwin H. Land, on February 10, 1948. In general, such composite film units comprise a photosensitive element such as the integral multilayer element heretofore mentioned, an image-receiving element, and a rupturable pod containing an aqueous alkaline processing composition. The photosensitive element, image-receiving element and pod are so associated with each other that, upon processing, the photosensitive element may be superposed on the image-receiving element, and the pod may be ruptured to spread the aqueous alkaline processing solution between the superposed elements. The nature and construction of the I pods and such composite film units are well known in the 8 art; see, for example, US. Patents Nos. 2,543,181 and 2,634,886, issued to Edwin H. Land, as well as the previously noted copending United States applications Serial Nos. 748,421 (now U.S. Patent No. 2,983,606, issued May 9, 1961) and 565,135.

While the invention has been illustrated in connection with integral multilayer negatives having a plurality of superposed silver halide emulsion layers and the invention is particularly applicable to such units, it should be understood that in certain instances, use of the delayed availability of a development restrainer as contemplated in this invention may be advantageous in processes employing other types of photosensitive elements, e.g., a screen type photosensitive element, such as disclosed in the previously noted copending United States application Serial No. 748,421, wherein at least two selectively sensitized silver halide emulsions are arranged in the form of a screen and the color-providing substances, as in multilayer photosensitive elements, are preferably placed in a separate alkali-permeable layer behind the silver halide emulsion with which they are associated.

The use of a development restrainer, made available after a predetermined period, may be advantageously used to increase the maximum density of the transfer image in monochrome processes or multicolor processes employing a plurality of separate sets of silver halide emulsions and image-receiving layers.

It is also contemplated to make a development restrainer available after a predetermined period by hydrolysis of a suitable derivative of the development restrainer. In such an embodiment, the hydrolyzable development .restrainer is preferably substantially non-diffusible, and at least substantially less dilfusible in its unhydrolyzed form than in its hydrolyzed form. Hydrolysis of the hydrolyzable development restrainer after a predetermined induction period will thus be an effective way of controlling the availability of the development restrainer and insuring that development is carried out unimpeded by the development restrainer for at least a time sufficient to develop the exposed and developable silver halide to the minimum extent necessary to properly modulate the appropriate dye developer. As examples of suitable hydrolyzable groups, mention may be made of oxalates having various length alkyl groups to obtain the desired induction period prior to hydrolysis and availability of the development restrainer. In such embodiments, the hydrolyzable development restrainer may be located much closer physically to the appropriate silver halide emulsions than if such development restrainers were initially diffusible.

It should be understood that while the image-receiving layer is preferably located so as to be superposed with the outermost or blue-sensitive silver halide emulsion layer of the photosensitive element and to be carried by its own support, it is also contemplated to have the image-receiving layer carried by the same support as are the photosensitive layers, i.e., on the same side or on the opposite side of the support as the silver halide emulsion layers. When the image-receiving layer is carried on the same side of the support as are the emulsion layers, the developed and processed emulsion layers are removed from the image-receiving layer by use of suitable techniques, e.g., by the use of appropriate stripping layers. Where the image-receiving layer is located on the opposite side of the support from the photosensitive layers, it may be unnecessary to remove the emulsion layers if the support is opaque or the photosensitive layers are otherwise treated to render them harmless to the viewing of the positive or transfer image contained in the image-receiving layer.

Since certain changes may be made in the above processes and products without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process of forming a multicolor image by diffusion transfer comprising (a) exposing a photosensitive element comprising a support carrying, in turn, a layer including a red-sensitive silver halide emulsion, a layer including a green-sensitive silver halide emulsion and a layer inclu-ding a blue-sensitive silver halide emulsion, said silver halide emulsions having associated therewith, respectively, a cyan dye developer, a magenta dye developer and a yellow dye developer, (b) applying an aqueous alkaline processing solution to said photosensitive element to initiate development of exposed silver halide, said dye developers being oxidized and thereby immobilized where development occurs, (c) continuing said development of exposed silver halide for a predetermined period, said predetermined development period being sufficiently long to form an i-magewise distribution of diffusible unoxidized dye developer in undeveloped areas of each of said silver halide emulsion layers as a function of said development, said unoxidized dye developers being transferred, by diffusion, to an image-receiving layer in superposed relationship with said silver halide emulsion layers to form said multicolor image, (d) said process including the step of restraining further development of exposed silver halide after said predetermined development period, said restraining being effected by diflusing to at least one of said silver halide emulsion layers a reagent which renders said silver halide substantially less developable, said reagent being initially contained in a layer of said photosensitive element other than a layer containing a silver halide emulsion, whereby said multicolor transfer image exhibits greater color separation.

2. A process as defined in claim 1, wherein said predetermined period is about to seconds.

3. A process as defined in claim 1, wherein each of said dye developers is initially contained in a layer positioned between its associated silver halide emulsion layer and sa-id support.

4. A process as defined in claim 1, wherein said photosensitive element includes an alkali-permeable layer containing a benzenoid silver halide developing agent, said alkali-permeable layer being coated over said blue-sensitive emulsion layer.

5. A process as defined in claim 1, wherein said reagent forms a substantially insoluble reaction product with unexposed and-undeveloped silver halide as well as with exposed but undeveloped silver halide.

6. A process as defined in claim 1, wherein said predetermined development period is controlled by the diffusion rate of said reagent through said processing solution.

7. A process as defined in claim 1, wherein said reagent is released during development by alkaline hydrolysis of a hydrolyzable derivative of said reagent.

8. A process as defined in claim 1, wherein said aqueous alkaline processing solution contains an antifoggant eflt'ective to minimize the development of unexposed silver halide during said predetermined period.

9. A process as defined in claim 1, wherein said reagent is S-nitrobenzimidazole.

10. A process of forming a multicolor image by diffusion transfer comprising (a) exposing a photosensitive element comprising a support carrying, in turn, a layer including a red-sensitive silver halide emulsion, a layer including a green-sensitive silver halide emulsion and a layer including a blue-sensitive silver halide emulsion, said silver halide emulsions having associated therewith, respectively, a cyan dye developer, a magenta dye developer and a yellow dye developer, (b) applying an aqueous alkaline processing solution to said photosensitive element to initiate development of exposed silver halide, said dye developers being oxidized and thereby immobilized where development occurs, (0) continuing said development of exposed silver halide for a predetermined period, said predetermined development period being sufiiciently long to form an imagewise distribution of difiiusible unoxidized dye developer in undeveloped areas of each of said silver halide emulsion layers as a function of said development, said unoxidized dye developers being transferred, by diffusion, to an image-receiving layer in superposed relationship with said silver halide emulsion layers to form said multicolor image, (d) said process including the step of restraining further development of exposed silver halide after said predetermined development period, said restraining being effected by diffusing to at least one of said silver halide emulsion layers a reagent which renders said silver halide substantially less developable, said reagent being initially contained in a separate element containing said superposed image-re ceiving layer, whereby said multicolor transfer image exhibits improved color separation.

11. A process as defined in claim 10, wherein said predetermined period is about 5 to 20 seconds.

12. A process as defined in claim 10, wherein each of said dye developers is initially contained in a layer positioned between its associated silver halide emulsion layer and said support.

13. A process as defined in claim 10, wherein said photosensitive element includes an alkali-permeable layer containing a benzenoid silver halide developing agent, said alkali-permeable layer being coated over said bluesensitive emulsion layer.

14. A process as defined in claim 10, wherein said reagent forms a substantially insoluble reaction product with unexposed and undeveloped silver halide as well as with exposed but undeveloped silver halide.

15. A process as defined in claim 10, wherein said predetermined development period is controlled by the diffusion rate of said reagent through said processing solution.

16. A process as defined in claim 10, wherein said reagent is released during development by alkaline hydrolysis of a hydrolyzable derivative of said reagent.

17. A process as defined in claim 10, wherein said aqueous alkaline processing solution contains an antifoggant efiective to minimize the development of unexposed silver halide during said predetermined period.

18. A process as defined in claim 10, wherein said reagent is thioacetanilide.

References Cited by the Examiner UNITED STATES PATENTS 2,636,821 4/1953 Sargent 96-22 2,725,290 11/ 1955 Smith 96-22 2,983,606 5/1961 Rogers 9629 2,997,390 8/ 1961 Land. 3,019,108 1/ 1962 Dershowitz 96-66 3,039,869 6/ 1962 Rogers et al. 963 3,043,692 7/ 1962 Haas et al 9629 FOREIGN PATENTS 221,880 5/ 1959 Australia.

569,080 1/1959 Belgium.

804,972 11/ 1958 Great Britain.

OTHER REFERENCES Glafkides: Photographic Chemistry, Fountain Press, London (1958) pp. 369-390.

Glass et al.: P.S.A. Technical Quarterly, November 1956, p. 167.

Ilford: Manual of Photography, Ilford, England, (1958) pp. 395-403.

Mees: Theory of the Photographic Process, Mac- Millan, New York, (1954) pp. 677-680.

NORMAN G. TORCHIN, Primary Examiner.

PHILIP E. MANGAN, Examiner.

BERNARD E. EDELSTEIN, GERALD H. BJORGE,

J. TRAVIS BROWN, Assistant Examiners. 

1. A PROCESS OF FORMING A MULTICOLOR IMAGE BY DIFFUSION TRANSFER COMPRISING (A) EXPOSING A PHOTOSENSITIVE ELEMENT COMPRISING A SUPPORT CARRYING, IN TURN, A LAYER INCLUDING A RED-SENSITIVE SILVER HALIDE EMULSION, A LAYER INCLUDING A GREEN-SENSITIVE SILVER HALIDE EMULSION AND A LAYER INCLUDING A BLUE-SENSITIVE SILVER HALIDE EMULSION, SAID SILVER HALIDE EMULSIONS HAVING ASSOCIATED THEREWITH, RESPECTIVELY, A CYAN DYE DEVELOPER, A MAGENTA DYE DEVELOPER AND A YELLOW DYE DEVELOPER, (B) APPLYING AN AQUEOUS ALKALINE PROCESSING SOLUTION TO SAID PHOTOSENSITIVE ELEMENT TO INITIATE DEVELOPMENT OF EXPOSED SILVER HALIDE, SAID DYE DEVELOPERS BEING OXIDIZED AND THEREBY IMMOBILIZED WHERE DEVELOPMENT OCCURS, (C) CONTINUING SAID DEVELOPMENT OF EXPOSED SILVER HALIDE FOR A PREDETERMINED PERIOD, SAID PREDETERMINED DEVELOPMENT PERIOD BEING SUFFICIENTLY LONG TO FORM AN IMAGEWISE DISTRIBUTION OF DIFFUSIBLE UNOXIDIZED DYE DEVELOPER IN UNDEVELOPED AREAS OF EACH OF SAID SILVER HALIDE EMULSION LAYERS AS A FUNCTION OF SAID DEVELOPMENT, SAID UNOXIDIZED DYE DEVELOPERS BEING TRANSFERRED, BY DIFFUSION, TO AN IMAGE-RECEIVING LAYER IN SUPERPOSED RELATIONSHIP WITH SAID SILVER HALIDE EMULSION LAYERS TO FORM SAID MULTICOLOR IMAGE, (D) SAID PROCESS INCLUDING THE STEP OF RESTRAINING FURTHER DEVELOPMENT OF EXPOSED SILVER HALIDE AFTER SAID PREDETERMINED DEVELOPMENT PERIOD, SAID RESTRAINING BEING EFFECTED BY DIFFUSING TO AT LEAST ONE OF SAID SILVER HALIDE EMULSION LAYERS A REAGENT WHICH RENDERS SAID SILVER HALIDE SUBSTANTIALLY LESS DEVELOPABLE, SAID REAGENT BEING INITIALLY CONTAINED IN A LAYER OF SAID PHOTOSENSITIVE ELEMENT OTHER THAN A LAYER CONTAINING A SILVER HALIDE EMULSION, WHEREBY SAID MULTICOLOR TRANSFER IMAGE EXHIBITS GREATER COLOR SEPARATION. 